This invention relates to a method for manufacturing a semiconductor device and in particular a method for manufacturing a semiconductor device which includes an ion implantation step.
In the manufacture of a semiconductor device, a step of forming a desired impurity-region by ion implantation is adopted to obtain a high performance semiconductor device. This step is carried out by, for example, forming a buffer oxide film on a semiconductor substrate, coating a resist film on the oxide film, subjecting the resist film to patterning by photolithography to form a window corresponding to an impurity-region formation region and ion implanting a desired impurity into a resultant structure with the resist film pattern as a mask. In this method, the surface of the semiconductor substrate is even and flat after the removal of the resist film subsequent to the formation of the impurity-region. In any subsequent step, no mask alignment mark remains for the formation of a resist film pattern, etc. It is therefore conventionally necessary to provide a step for forming a mask alignment mark using one of the methods set out below.
(1) Before the step of forming a mask for ion implantation, a mask alignment mark which is necessary for photolithograhy in the subsequent step is formed on the surface of the buffer oxide film or the semiconductor substrate using another mask.
(2) After the formation of the resist film pattern as the mask for ion implantation, openings are formed with the resist film pattern as the mask in the oxide film underlying the resist film such that they reach the semiconductor substrate underlying the oxide film. The opening is used as an alignment mark in the subsequent step for photolithography.
According to method (1), the number of photolithography steps is increased, which results in a high manufacturing cost and an accuracy problem with respect to the configuration of the pattern.
In method (2), implanting an impurity through the opening directly into the semiconductor substrate is liable to produce a crystal defect. It is very difficult to control the depth of etching in the formation of the opening. Furthermore, a crystal defect is liable to be produced.
When the impurity-region is formed by the above-mentioned ion implantation method, the resultant semiconductor device has a high noise level.